Dopamine, a major catecholamine neurotransmitter, is involved in the regulation of a variety of functions which include emotion, cognition, motor functions, and positive reinforcement, (Purves, D. et al. (2004) Neuroscience. Sinauer, third edition, Sunderland, Mass.).
The biological activities of dopamine are mediated through G protein-coupled receptors (GPCRs) and in human, five different dopamine receptors D1-D5 have been identified, where the D2-like receptors (D2, D3 and D4) couple to the G-protein GαI (Missale, C. et al. (1998) Dopamine receptors: from structure to function. Physiol. Rev. 78, 189-225). The D3 dopamine receptor is most highly expressed in the nucleus accumbens (Gurevich, E. V., Joyce, J. N. (1999) Distribution of dopamine D3 receptor expressing neurons in the human forebrain: comparison with D2 receptor expressing neurons. Neuropsychopharmacology 20, 60-80), and is proposed to modulate the mesolimbic pathway consisting of neuronal projections from the ventral tegmental area, hippocampus and amygdala to the nucleus accumbens, which projects to the prefrontal and cingulate cortices as well as various thalamic nuclei. The limbic circuit is thought to be important for emotional behavior and thus D3 receptor antagonists are proposed to modulate psychotic symptoms such as hallucinations, delusions and thought disorder (Joyce, J. N. and Millan, M. J., (2005) Dopamine D3 receptor antagonists as therapeutic agents. Drug Discovery Today, 1 July, Vol. 10, No. 13, 917-25), while these antagonists spare the D2 modulated striatal extrapyramidal system (associated with EPS induction). In addition, it has been reported that drug naive schizophrenic patients show altered levels of D3 receptor expression (Gurevich, E. V. et al. (1997) Mesolimbic dopamine D3 receptors and use of antipsychotics in patients with schizophrenia. A postmortem study. Arch. Gen. Psychiatry 54, 225-232) and dopamine release (Laruelle, M. (2000) (2000) Imaging dopamine dysregulation in schizophrenia: implication for treatment. Presented at Workshop Schizophr.: Pathol. Bases and Mech. Antipsychotic Action, Chicago), indicating that a disturbed homeostasis of dopamine plays an important role in the etiology of schizophrenic symptoms.